The present invention relates to apparatus and method for exposing curable material to high intensity ultraviolet light.
Of particular relevance to the present invention, are systems and methods by which curable material forms part of a protective or decorative surface on objects that, typically are of a size which can be held in a human hand, and which are to be processed at a relatively high rate. This processing can be of a high speed batch type, whereby each object having curable material thereon, is fixtured in spaced relation from a series of other such objects, and conveyed to a particular location where each object is subjected to high intensity ultraviolet (UV) curing light. The light passes through a shutter which opens and closes to control the duration of the curing exposure. Such conveying can be implemented by an indexing conveyor, such as a rotary indexing table of the type described in pending U.S. patent application Ser. No. 08/668,445 filed Jun. 20, 1996 for "Rotary Indexing Table", now U.S. Pat. No. 5,784,932 the disclosure of which is hereby incorporated by reference.
Indexing conveyors have been used in the decorating field for screen printing, hot stamping, pad printing, ink-jet printing, impact marking, laser marking, spray painting and other decorative purposes. As one particular example, rotary indexing tables are employed for multi-color screen printing onto work pieces such as compact discs, credit cards, key fobs etc. Typically, the dial of a rotary indexing table supports multiple, equidistantly positioned fixtures. The fixtures support the work pieces during the printing and curing operations. At a first work station (or section of a work station), a work piece is automatically positioned onto the fixture. The dial then rotates through a precise angle or distance to position the work piece under a first screen printing apparatus. The dial is thereafter indexed one more increment such that the work piece is positioned beneath a source of UV light. A shutter is opened, the work piece is exposed for a preset time interval during the dwell time of the indexing conveyor, and the shutter is then closed. The dial is then indexed for the screen printing of a subsequent pattern or image, and indexed again for another UV curing step, whereby the process continues until all the printing and curing has been completed for a particular work piece.
In the particular example of the work piece being a compact disc, the work piece is generally flat, with a length and width that are measured in inches, e.g., a compact disc is typically about 5 inches in diameter.
In conventional equipment of the type employed for such UV curing, the shutter system may have single or dual slide gates, single or dual pivoting reflectors, or so-called "bombay doors". For example, if two bombay doors open and close across a six by six area, each door is rotated 90.degree. downward to a substantially vertical orientation, for opening the shutter. Of course, the doors would then each rotate 90.degree. back to the horizontal, for closing the shutters. Therefore, the exposure of the work piece below the shutter would have a duration determined in part by the time required for the doors to cycle 180.degree., i.e., actuate 90.degree. to full open, then actuate 90.degree. to full closure.
One can readily appreciate that with an indexing conveyor system, whereby a series of work pieces are conveyed beneath the shutter, so that an individual work piece periodically is located in a stationary condition for a pre-determined time interval, the center of the work piece (which lies directly under the juncture of the bombay or other conventional doors), will receive significantly more UV exposure than the outer surfaces of the work piece. This can present a problem because curing of UV inks or adhesives, requires a precise and even dosage of light energy for optimum results. Moreover, not only do the bombay doors and other conventional types of shutters produce uneven exposure, but the speed at which the shutters can operate through a full cycle, is limited by the need to mechanically accelerate and decelerate one or two relatively larger shutter elements through 180.degree. of angular motion (i.e., 90.degree. clockwise to open and then 90.degree. counter-clockwise to close), or through a complete reversal of linear sliding.